Why is the sky blue?

Why is the sky blue?#

The daytime sky on Earth has a bluish color due to a physical phenomenon called the scattering of sunlight by molecules present in the atmosphere. The light arriving directly from the Sun can look orange, while the light scattered by air molecules gives the sky its blue appearance.

When we look at the sky, we are seeing only the sun rays that were deflected by the molecules of the atmosphere in such a way that they are directed straight to our eyes. Certain particles and molecules in the atmosphere have the ability to diffuse solar radiation in all directions.

In the absence of an atmosphere, the sky appears black, and stars and other celestial bodies stand out clearly. In the presence of an atmosphere, during the day, the Earth's sky appears bluish, and the scattering of light establishes an average light intensity that normally outshines other celestial bodies.

How does sunlight interact with the atmosphere?#

The white light of the sun is a mixture of all the colors of the rainbow, ranging from red to violet. The atoms and molecules of the atmosphere, such as oxygen and nitrogen, are small in size and diffuse short-wavelength light, which corresponds to blue and violet, more efficiently.

Almost all red rays coming from the Sun pass through the atmosphere without difficulty. It is the blue and violet ones that get deflected. As a result of this physical phenomenon, the Sun has a more reddish-yellow color than when observed outside the Earth's atmosphere.

Each photon that bounces encounters other molecules and can bounce again in another direction. They end up reaching the Earth's ground coming from everywhere, from random directions. Therefore, no matter which way we look, we see scattered blue photons.

Read also: How does a refrigerator manage to cool food?

What is the role of Rayleigh scattering?#

The physical mechanism causes shorter wavelength light to be scattered with much greater intensity, varying approximately with the inverse of the fourth power of that wavelength. The phenomenon honors the British physicist Lord Rayleigh, who deeply studied the color and polarization of celestial light.

Rayleigh published works on the color and polarization of sky light. He related the scattered intensity to the size and refractive index of small particles and subsequently derived the result from electromagnetic theory to consolidate the explanation.

Why does the sunset turn reddish?#

At dawn or dusk, sunlight has to travel a much longer path through the atmosphere before reaching our eyes. As a result of this prolonged journey, the sun and the region near it lose a large part of their bluish components.

When the air contains small particles of salt or dust in the lower layers, even green and yellow photons are diffused more effectively. This causes celestial bodies and the horizon to take on a striking orange or reddish color.

When the air is clean and the Sun or Moon is rising or setting near the horizon, they have a yellowish color because the light rays have to travel a much greater distance through the atmosphere before reaching our eyes, and many more blue photons are diffused.

Frequently asked questions#

Why doesn't the sky look violet if violet has a shorter wavelength?#

Although violet has a shorter wavelength and undergoes heavy scattering, the amount of this radiation in sunlight is lower, and human vision possesses receptors less sensitive to this color. The visual system interprets the mixture by stimulating the blue cones and other receptors, resulting in the perception of pale blue.

Is the sky blue on other planets?#

Not necessarily. On Mars, for example, dust suspended in the atmosphere often makes the sky look yellowish or reddish, demonstrating that the color depends on the composition of the atmosphere and the size of the particles present.

Why is the sky black at night?#

At night, the sky has a black appearance because there is no direct sunlight to undergo the process of diffusion or scattering by the molecules present in the Earth's atmosphere.

Are clouds also blue?#

No. Clouds are white because they consist of droplets with diameters larger than the wavelength of visible light, which causes them to diffuse all wavelengths in an approximately equal manner.

Keep reading#

• How does a refrigerator manage to cool food?
• Why does ice float on water?
• How does a microwave heat food?

Sources

1. Rayleigh scattering (Wikipedia)
2. Sky (Wikipedia)